The International Mathematical Olympiad (IMO) is a six-question two-day mathcompetition for students that have not yet gone to college and are under 20 years old. Any country may take part in it by sending a team of six students to compete.
History
The IMO has been held every year since 1959. It takes place in a different country (which is called the "host country") from year to year. However, it is often seen that this competition is dominated by the US and China.